1. Field of the Invention
The present invention relates to an electroless gold plating liquid, and more specifically, to an electroless gold plating liquid which may form a gold plating without corrosion of a base metal by performing substitution and reduction reactions in the same bath, and satisfy both weldability of lead-free soldering and wire bonding characteristics, and has excellent stability such that a gold deposition rate may be continuously maintained.
2. Discussion of Related Art
In order to join components, a printed circuit board is gold plated. The gold plating on the printed circuit board is the final process performed in the manufacturing process in order to prevent oxidation of a pad surface, and because gold plating largely affects mounting properties, soldering properties, etc. of components, it has a large influence on reliability of components.
Recently, as circuits are being more highly integrated and refined, a method of electroplating gold requiring conduction of electricity is being replaced with an electroless gold plating method.
The electroless gold plating method includes a reductive plating method of plating with deposition through autocatalysis by a reductant, and a displacement plating method of substituting gold for a base metal. In the case of the autocatalytic electroless gold plating method using the reductant, the use thereof is limited because a thickness of the gold plating is insufficient, and in the case of the displacement plating method, the use thereof is limited because corrosion is generated in the base metal and a thickness of the gold plating is insufficient. In the case of the autocatalytic electroless gold plating method using the reductant, adhesion force of the plated gold becomes non-uniform, and thus weldability may not be ensured upon lead-free soldering.
Many studies on increasing adhesion force of the plating while suppressing corrosion of the base metal and improving stability of an electroless gold plating liquid have been performed. From studies in search of a proper reductant or in which a metal elution inhibitor was added in the electroless gold plating method, methods using ascorbic acid (Japanese Laid-Open Patent Publication No. 1989-191782), hydrazine compounds (Japanese Laid-Open Patent Publication No. 1991-215677), thiourea (Japanese Laid-Open Patent Publication No. 1997-287077), and phenyl-based compounds (Japanese Patent Publication No. 2972209) as the reductant, and methods using benzotriazole-based (Japanese Laid-Open Patent Publication No. 1992-314871), mercaptobenzothiazole-based (Japanese Laid-Open Patent Publication No. 1992-350172), and hydroquinone-based compounds (Japanese Laid-Open Patent Publication No. 2003-268559) as the metal elution inhibitor were introduced.
Further, U.S. Pat. No. 6,855,191 discloses a method using 2-mercaptobenzothiazole as a stabilizer, U.S. Pat. No. 6,383,269 discloses a method using hydroxylamine compounds as a reductant, U.S. Pat. No. 5,935,306 discloses a method using ascorbic acid or salts thereof as a reductant, and U.S. Pat. No. 5,601,637 discloses a method using sodium nitrobenzene sulfonate and/or para-nitrobenzoic acid as an oxidizer to control a reduction rate.
However, there was a limitation in addressing the problems such as maintenance of the deposition rate, stability of a plating bath, adhesion force of the plating, corrosion of the base metal, or the like, and thus studies on a more stable plating bath, continuous maintenance of the deposition rate, prevention of corrosion of the base metal, and increase of adhesion force have continued, and electroless gold plating methods in which a water-soluble amine compound is further added in addition to a water-soluble gold compound, a complexing agent, and a reductant which form an electroless gold plating were studied and have seen much progress.
The conventional art for this electroless gold plating method adding the water-soluble amine compound is as below.
In Korea Patent Application No. 2003-0045071, an ethylenediamine compound, which is the most effective among the above-described compounds, is used as a kind of water-soluble amine, and hydroquinone, methylhydroquinone, or the like are used as a phenyl-based compound.
In Korea Patent Application No. 2006-0016767, ethylenediamine or glycine is used as a kind of water-soluble amine, and a hydroxy alkyl sulfonic acid or salt is used as a reductant.
In Korea Patent Application No. 2008-0066570, an ethylenediamine derivative is used as a kind of water-soluble amine, and formaldehyde bisulfite is used as a reductant.
In Korea Patent Application No. 2012-0031990, polyethyleneamine is used as a kind of water-soluble amine, and borohydride and a boron compound are used as a reductant.
However, in the above-described conventional art, although using a water-soluble amine compound may slow down a displacement reaction rate, corrosion and pit generated in the base metal were not completely prevented, and weldability of lead-free soldering was not ensured at a lead-free soldering temperature in a range of 250 to 260° C.
Recently, due to prohibitions on the use of Sn/Pb solder, plating methods have been changed to soldering methods using lead-free (Sn/Ag/Cu) solder, soldering temperatures have been raised to a range of 250 to 260° C., thermal loads have increased, and thus the electroless gold plating having reinforced characteristics has been required to overcome the thermal load.
With the use of lead-free (Sn/Ag/Cu) solder, the electroless gold plating method of ENEPIG (electroless nickel/electroless palladium/immersion gold) method is required.
This method is a method of plating electroless palladium between plating of electroless nickel and plating of electroless gold to prevent oxidation and diffusion of nickel, improve corrosion resistance of circuits or terminals, and overcome degradation of bonding characteristics of nickel and gold plating. In the ENEPIG method, a potential difference between palladium and gold is small, and thus uniform gold plating on a palladium-plated surface is hard to obtain using the existing electroless gold plating liquid, and a desired thickness of the gold plating is also hard to obtain.
Further, the gold plating having a thickness of 0.05 μm or more needs to be formed on the palladium plating layer for lead-free (Sn/Ag/Cu) soldering, and the gold plating having a thickness of 0.25 μm or more needs to be formed on the palladium plating layer for wire bonding. When lead-free (Sn/Ag/Cu) soldering and wire bonding are performed at the same time, the gold plating having a thickness of 0.25 μm or more needs to be plated on the palladium plating layer.
When a general displacement plating method or reduction gold plating method is used, corrosion of the base metal may not be prevented, and weldability of lead-free (Sn/Ag/Cu) soldering and wire bonding characteristics are insufficient. At the same time at which a substitution reaction starts, continuation of the substitution reaction should be stopped, the reaction should be converted to a reduction reaction immediately, and a uniform gold plating surface and the gold plating having a sufficient thickness should be obtained. Continuation of the substitution reaction may be stopped by catalyzing palladium separated from the palladium plating through an ionic catalyst and inducing gold plating at the same time at which the substitution reaction starts, and thus an ionic catalyst activator capable of converting palladium to an ionic catalyst material is needed.